Difficulties attendant with obtaining completely sporulated oocysts from in vitro cultures means that oocysts and sporozoites must still be obtained from infected animals. The greatest numbers can most easily be obtained from infected sulking aged calves. Techniques for oocyst preservation, isolation and purification are well established. Recently, techniques have been described and improved upon for the isolation of merozoites from infected animals. Improvements of in vitro cultivation techniques will likely further impact on the isolation of this asexual life cycle stage. Core 9001 of the Program Project is designed specifically to provide parasite reagents including oocysts, sporozoites, and merozoites of Cryptosporidium parvum for use in Projects #1-4 and core 9002 of the Program Project as prescribed by the needs of each. Without this Core, there would be no Program Project.